Both would be on the ragged edge of what your 2100 will handle. I've got a 1998 B2400 and pull a 5ft BB with it, but need low range if the rippers are involved. I usually run out of traction before engine bogs down in low.
I've never tried a 5ft cutter behind mine, but do have a 4ft King Kutter that I previously used behind a B7100. When cutting deep, heavy cover, I can load the engine pretty good with it. Long uphill mowing also tends to raise temperature. I would say you'll get by with a 5ft if your not cutting anything very heavy, and will just have to take it slowly.